Integrated circuits typically include output circuits for driving data signals onto buses or input/output (I/O) pins. These output circuits generally have three states, namely a high state, a low state and a high-impedance state. The output circuit enters the high and low states in response to a data (d) signal when enabled, and enters the high impedance state in response to an output-enable (oe) signal. For low voltage TTL technology, in the high state, the output circuit provides a logic high output signal (e.g., a voltage of greater than 2.4 volts) at an output node. In the low state, the output circuit provide a logic low output signal (e.g., a voltage of less than 0.4 volts) at the output node. In the high-impedance state, the output circuit presents a high impedance (e.g., four hundred K.OMEGA. or greater) to the output node to allow other output circuits to drive the bus or I/O pin.
A typical three-state output circuit includes a pull-up transistor and a pull-down transistor connected with their channel regions in series between a supply voltage source (POWER) and a ground source (GROUND). The node connecting the channel regions of the pull-up and pull-down transistors serves as the output node. When the output circuit is in the high state, the pull-up transistor is conductive while the pull-down transistor is non-conductive, thereby electrically connecting the output node to POWER. In contrast, when the output circuit is in the low state, the pull-down transistor is conductive while the pull-up transistor is non-conductive, thereby electrically connecting the output node to GROUND. However, when the output circuit is in the high-impedance state, both the pull-up and pull-down transistors (i.e., the output transistors) are in the non-conductive state. The output circuit typically enters the high-impedance state when the oe signal is de-asserted.
However, when a conventional output circuit transitions from the high state to the low state (and vice versa), both of the output transistors can be conductive. When both of the output transistors are conductive, a direct current path is formed between the supply and ground sources through the pull-up and pull-down transistors. This condition is undesirable because it allows a current (i.e., the "crow-bar" current) to flow between POWER and GROUND, thereby wasting power and, in extreme cases, introducing switching noise in the power supply buses or damaging the output transistors.